1. Field of the Invention
The present invention relates to piston rings mounted in pistons of internal combustion engines.
2. Description of the Related Art
In recent years, the operating environment of the piston ring has become more severe along with exhaust emission regulations and demands for higher engine output. In piston rings in particular, having a hard film to improve durability formed by ion plating over a nitrided base material of martensitic stainless steel, the problem of a drop in fatigue strength occurs. As a countermeasure, the Japanese Patent Laid-open No. 5-172248 and Japanese Patent Publication No. 6-25597 (Japanese Patent Laid-open No. 62-120471) disclose a piston ring forming a nitrided layer only on the upper and lower surfaces and inner circumferential surface of a steel piston ring and covering the outer circumferential surface with a hard film by ion plating. However, these official gazettes mention nothing of a hard film formed by ion plating and nitrided layer not in contact with each other in the vicinity of the corners formed by the upper and lower surfaces and outer circumferential surface of the piston ring.
The above-mentioned piston rings of the known art have been designated as having insufficient fatigue strength under severe engine operating conditions.
The present invention is a piston ring formed with a nitrided layer on upper and lower surfaces or upper and lower surfaces and an inner circumferential surface of the piston ring, and formed with a hard film by ion plating on an outer circumferential surface of the piston ring, wherein the nitrided layer has a Vickers hardness of 700 or more on at least one of the upper and lower surfaces and the hard film formed by ion plating on the outer circumferential surface are separated and are not in contact with each other.
Piston rings under harsh engine operating conditions are subjected in particular to bending stresses. This bending stress is applied in particular to the comer sections on the outer circumferential side of the piston ring. In an engine with high combustion pressure, a load is applied to the corner section on the lower surface side of the piston ring, and in an engine at high speed, a load is applied to the corner section on the upper surface side of the piston ring due to the occurrence of fluttering. However, this load can be alleviated by setting a nitrided layer with a Vickers hardness of 700 or more and a hard film formed by ion plating not in contact with each other, on at least one side of the upper and lower surfaces in the corner sections on the outer circumferential side of the piston ring, and a piston ring with improved fatigue strength can be obtained. Which side is chosen between the upper and lower surfaces in the corner sections of the outer circumferential side of the piston ring as the side where the nitrided layer with a Vickers hardness of 700 or more and the hard film formed by ion plating do not make contact, varies according to the type of engine, however, it is preferable not to make contact on both sides of the upper and lower surfaces.
The distance between the nitrided layer with a Vickers hardness of 700 or more and the hard film formed by ion plating is preferably 0.001 mm or more for improved fatigue strength and preferably 0.3 mm or less for good wear resistance. When this distance is larger, the wear on this portion increases, so a distance of 0.001 to 0.1 mm is even more preferable.
The material of the piston ring is preferably martensitic stainless steel.
The nitrided layer is formed by a method such as gas nitriding or salt bath softnitriding. The nitrided layer is preferably formed by the salt bath softnitriding method in order to minimize deformation of the ring. The nitrided layer with a Vickers hardness of 700 or more preferably has a thickness in a range from 1 to 110 xcexcm. Ring deformation increases as the nitrided layer becomes thicker and ring wear on the upper and lower surfaces increases as the nitrided layer becomes thinner, so the nitrided layer with a thickness in a range from 3 to 40 xcexcm is even more preferable.
The hard film formed by ion plating is comprised of a CrN, Cr2N, mixture of CrN and Cr2N, mixture of Cr and either or both of CrN and Cr2N, or TiN, etc. At least one of oxygen and carbon is preferably contained in a solid solution state in the crystal of the film.
The thickness of the hard film formed by ion plating is preferably within a range from 1 to 80 xcexcm.